Field of the Invention
The invention relates to a method of producing a platinum-metal structure or pattern by a lift-off process, more particularly, on a substrate and in the course of producing an integrated circuit.
In semiconductor technology, the use of platinum metals as conductive materials is becoming of interest. In this context, "platinum metal" should be understood to be any of the group of platinum metals, namely Pt, Ir, Ru, and so forth. Being precious or noble metals, they have a high conductivity and, even at high temperatures in an oxygen-containing atmosphere, will not form nonconductive oxides. Moreover, they have good interface properties, both in mechanical and in electrical terms, with respect to many substances used in semiconductor technology. An important field of application of platinum metals is the use thereof as an electrode material in storage capacitors in integrated DRAM memories, in particular, if the latter have a so-called high-.epsilon. dielectric (for example BST) or a ferroelectric as the storage dielectric. The fabrication of these storage dielectrics requires high temperatures in an oxidizing atmosphere. Such storage cells require little space or have a relatively small footprint and are of great interest for future generations of memories.
A drawback of the platinum metals is that they cannot be etched readily. Methods to date are based predominantly upon physical etching, for example ion milling, and therefore have only limited selectivity with respect to other materials such as photoresist, for example. Lift-off techniques employing a mask of photoresist or titanium have already been used heretofore for very large structures or patterns.
In a lift-off process, as is generally known, a mask is applied to a substrate under the layer to be structured or configured, the mask being formed with an opening at the locations of the pattern or structure to be produced. Only that part of the layer which overlies the mask is then removed together with the mask. In this context, it is important that the mask have a negative side at the edge of the opening, i.e., that the exposed substrate surface area be larger than the mask opening on the upper side of the mask. In other words, the mask must overhang in some form or other. As a result, the side of the mask is not coated, and the solvent for the mask can attack at this location. If photoresist is used as the mask in a liftoff process, these negative sides can be achieved only by special techniques which are unconventional for semiconductor fabrication (for example, by image reversal). If titanium is used as the mask, the time factor plays a vital role, because titanium is readily oxidized at the surface thereof upon contact with air and is then difficult to remove.